Aluminum alloys have enjoyed widespread use because of their high strength-to-weight ratios and therefore have been used extensively for mass-reduction efforts. This has been a dominant theme in the automotive industry where fuel economy and emissions reduction have motivated manufacturers to reduce mass to improve efficiency. As efficiency targets extend to higher levels, mass reduction has been coupled with power-density increases to meet requirements. However, higher power density drives higher loading and temperature in the service environment.
Historically, aluminum alloys and their heat treatments have been developed for room-temperature or near room-temperature applications. In materials science, an application of an alloy is considered elevated-temperature if the service environment includes any more than brief exposure above one-half the homologous melting temperature of the alloy. The homologous temperature is a fraction of the melting point on the absolute temperature scale (Aluminum TMP=660° C.+273=933 kelvin; 0.5 TMP=465.5 kelvin or 193.5° C.). Thus, any application above 194° C. is considered a high-temperature application. Above 0.5 TMP, different failure mechanisms become dominant in a component. For cylinder heads, the operating temperature routinely exceeds this value and in the near future, it is expected to increase another to between 0.55 and 0.58 TMP.
The high-volume commercial aluminum alloys are primarily strengthened by two mechanisms: work-hardening and precipitation-hardening. For applications which require mass-produced complex shapes, such as automotive engine components, work-hardened alloys are not practical or economical, leaving precipitation-hardening via heat treatment as the primary method to achieve required mechanical properties. Precipitation hardening is achieved through different heat treat steps that manipulate the microstructure such that very fine strengthening phases can be formed in a controlled manner by varying the time at temperature during the aging process. These strengthening mechanisms have been developed for systems and products intended for use at room temperature or at slightly elevated temperatures. However, once the temperature of the operating environment rises above typical aging temperature range of 150-220° C., the properties undergo rapid deterioration with increasing temperature and with increasing time at temperature.
Precipitation-hardening changes the mechanical properties of an aluminum alloy by precipitating clusters of atoms (“precipitates”) from a super-saturated solid solution of alloying elements in the parent aluminum phase. As the precipitates form, they distort the lattice, impeding the motion of dislocations. It is the impediment to dislocation motion that causes the change in properties; the hardness and strength increase and the ductility decreases.
The formation of precipitates is affected by time and temperature; at low temperatures, the precipitation reaction is sluggish and takes a large amount of time and at higher temperature, the reaction occurs more quickly due to higher atomic mobility.
At a given temperature, the strength and hardness increase with holding time at temperature until most of the potential second phase forms. With increased holding time, the individual precipitates undergo two fundamental changes; firstly, some particles grow at the expense of others. Through diffusion of alloying elements, some particles will shrink and eventually disappear whereas other will grow in size. This leads to a fewer number of larger precipitates. The larger distance between the fewer precipitates improves dislocation mobility leading to a decrease in hardness and strength and an increase in ductility. Additionally, as the precipitates grow in size, the strain energy between the precipitate and the aluminum lattice increases to a point where it becomes energetically feasible for the interface atomic bonds to be broken and form a separate phase boundary. This reduces the strain energy in two ways; the trans-boundary bonds are broken, allowing more separation and therefore less lattice distortion and since the crystal structures of the parent and precipitate lattices are different, they will no longer be forced to accommodate both sets of lattice parameters at the cluster-parent interface.
When the interface is still intact the distortion due to the disregistry is equal and opposite in the two phases. The zone of distortion reaches out beyond the chemical interface, disturbing the orderly arrangement of the lattice in the parent phase. This distortion allows the precipitate to have a disproportionately large impact on the mechanical properties. The effective radius of the precipitate is the chemical radius plus a fraction of the distortion zone because the distortion zone also impedes the motion of dislocations and dislocation account for the mechanical response of the material to a deformation load. The interface breaks down as the chemical radius increases in a gradual manner; first it becomes partial coherent, and then incoherent. At high levels of incoherency, the mechanical properties of the system begin to decrease with further precipitate growth because the effective radius of the precipitate is now decreasing due to a loss of lattice strain in the parent phase. The loss of effective radius coupled with the reduction in precipitate density described above, are accompanied by a loss in mechanical properties and conversely an increase in tensile ductility this phenomenon is known as over-aging.
Therefore, there is a need for improved castable aluminum alloy components and for methods of making them, especially at elevated temperature conditions.